The structure and absolute configuration of pseurotin (1), a new metabolite, isolated from culture filtrates of Pseudeurotium ovalis STOLK (Ascomycetes), has been shown to be 2‐[1′(S), 2′ (S)‐dihydroxhex‐3′‐ene‐yl]‐3‐methyl‐8(S)‐methoxy‐8‐benzoyl‐9(R)‐hydroxy‐(5S)‐1‐oxa‐7‐aza‐spiro[4.4]non‐2‐ene‐4, 6‐dione (1), by spectral data and chemical transformations, and by X‐ray analysis of its dibromo derivative 2 [1].

The structure and absolute configuration of pseurotin A, a new metabolite, isolated from culture filtrates of Pseudeurotium ovalis STOLK (Ascomycetes) has been shown to be (1'S,2'S, 5 S, 8 S. 9 R, 3'Z)-8-benzoyl-2 (l', 2-dihydroxy-3'-hexenyl)-9hydroxy-8-methoxy-3-methyl-1 -oxa-7-azaspiro (4.41non-2-ene-4.6-dione (1). 1.Introduction. -In 1968 Sigg et al. [ l ] isolated ovalicin from cultures of Pseudeurotium ovalis STOLK (Ascomycetes) as main metabolite and determined its structure. The biosynthesis [2] and the immuno-suppressive activity [3] of the new compound were studied. As reported in preliminary communications, we have found a second minor metabolite in the cultures of the same organism, named pseurotin A*). Its structure 1 was elucidated by spectral data and chemical transformations [4], and by X-ray analysis of its dibromo derivative 2 [5]. In this paper the details of this investigation are presented3). Spectral Data and Chemical Transformations of 1. -Pseurotin A (1) wasisolated from the culture broth of Pseudeurotium ovalis (strain S2269/F) by extraction with ethylene chloride. After final purification of the crude extracts by column chromatography on silica gel, the new metabolite was obtained from dichloromethane/hexane as colourless rhombic crystals of m.p. 162-163.5 and [ u ] g = -5+ 1" (c=0.5, methanol). The yield was ca. 30-35 mg/liter culture broth.Pseurotin A (I), a neutral substance, is readily soluble in polar solvents but much less soluble in non-polar solvents. Heating of solutions during a longer period of time caused decomposition of the compound. The molecular formula, C2*H2,NO,, was deduced from elemental analyses. In the mass spectrum the molecular ion is not observed. The highest peak at mlz 399 (M' -32) is due to the loss of methanol, as it is frequently observed with methyl ethers (especially with a-methoxy ketones) and methyl esters [6a] [6b]. The difference in mass of 32 I ) 2, 3,

The crystal structure of 12,13‐dibromopseurotin has been determined by single crystal X‐ray analysis. The crystals belong to space group P21 with a = 16.75, b = 9.63, c = 7.42 Å, β 95.9°, Z = 2. The structure was solved by the heavy atom technique and refined to R = 0.062 with 819 significant reflexions for 299 parameters.

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